Improvements in communication networks are made to provide higher transportation capacity and robustness. In modern networks, often there are multiple paths across network elements which can be used to increase bandwidth and overcome link and node failures. Robustness includes using network capacity optimally, rerouting around failure quickly, and providing transparency to affected network elements when rerouting changes are made. Various approaches in this regard involve the use of link aggregation.
Link aggregation refers to a process for operating a group of physical links as if they are a single link. FIG. 1A illustrates link aggregation as a network configuration and process used to aggregate multiple links. The link aggregation runs between a pair of network elements 120 and 122 in the network to enable transmission of user traffic on each of the links participating in Link Aggregation Group (LAG) 101. Aggregating multiple network connections in this fashion can increase throughput beyond what a single connection can sustain, and/or can be used to provide resiliency in case of a failure of one of the links. Basic link aggregation between two network elements has been standardized; see, e.g., Institute of Electrical and Electronics Engineers (IEEE) standard 802.1AX). Yet link aggregation is not limited to two network elements. For example, the Distributed Resilient Network Interconnect (DRNI) (see Clause 8 of IEEE 802.1AX-REV/D1.0) specifies extensions to link aggregation in order to be able to use link aggregation on a network interface even between more than two network elements.
Another extension of the basic link aggregation concept illustrated in FIG. 1A is multi-chassis link aggregation group (MC-LAG). An MC-LAG provides readily identifiable and reliable link aggregation across multiple separate network elements. FIG. 1B illustrates link aggregation over multiple chassis. Local network element 132 (referred to as C1) and peer network element 134 (referred to as C2) form a logical interface of MC-LAG 160. The other end of MC-LAG 160 is remote network element 151 (referred to as RC). For the view of remote network element 151 and other network elements within network 150, local network element 132 and peer network element 134 act as a single network element. C1 and C2 are coupled with each other through inter-peer link 138. Inter-peer link 180 contains a set of links and it serves as a conduit between C1 and C2 for exchanging control messages. In addition, it may contain enough bandwidth for traffic rerouting upon MC-LAG failure conditions. Note while FIG. 1B illustrates two network elements, C1 and C2 forming the logical interface of an MC-LAG, some MC-LAG contains more network elements for one logical interface. In other words, there may exist multiple peer network elements for one local network element in some MC-LAG.
MC-LAG may provide redundancy in a multi-chassis environment. When redundancy is provided, MC-LAG needs a graceful and speedy recovery mechanism upon link or network element failure. In addition, MC-LAG needs a robust mechanism to route traffic through the multi-chassis environment.